The present invention is related to an improved system and method for determining true ground speed in a vehicle, such as a locomotive or a transit vehicle, propelled by electric traction motors, and, more particularly, to a processor system and method for accurately determining true ground speed in the vehicle through the combined use of respective speed signals from a radar unit and other electromechanical speed sensors, such as tachometers.
Vehicles, such as locomotives, used for heavy haul applications are generally required to produce a substantially high level of tractive effort (adhesion) between the wheels of the vehicle and the rails on which the vehicle travels. The required high level of adhesion is achieved by controlling creep or wheel slip to within acceptable levels. As used herein creep or wheel slip is the difference between wheel speed and the vehicle speed. Thus, effective creep control requires accurate knowledge of the true ground speed of the locomotive.
Presently known techniques for computing or determining the true locomotive ground speed of the vehicle have generally used a true ground speed sensor, such as a radar unit, and have further used electromechanical speed sensors, such as tachometers, for obtaining wheel-axle speed values that are processed for estimating the locomotive true ground speed, which is used for controlling the creep of each axle. Generally, such techniques for computing the locomotive ground speed, have relied principally either on the radar signal for determining the value of the vehicle speed or, when such radar signal is not available or disabled, then the respective speed signals from the electromechanical sensors are used as backup. Although the radar signal may be substantially accurate when averaged over a sufficiently long period of time, the radar signal may also be generally susceptible to undesirable short term variations or oscillations, such as may occur due to radar dropouts and low frequency oscillations. The severity of the low frequency oscillations may depend on various characteristics of the locomotive, the equipment, and/or the environment in which the locomotive travels, such as vibration of the locomotive, rail track and/or terrain profile, the specific operating characteristics of the radar unit and manner of installation, etc. Similarly, the radar dropouts may occur due to the presence of snow on the ground, physical obstructions such as grade crossings, etc. In view of the above, the radar signal may be characterized as having a substantially accurate DC/average information content but not necessarily accurate instantaneous information. Conversely, the speed signals from the electromechanical sensors may be characterized as having substantially accurate instantaneous information of the wheel speed but not necessarily accurate long term stability due to creep.
Unfortunately, the presently known techniques for computing the true ground speed measurements used for creep control in the locomotive generally suffer from various drawbacks due to the foregoing undesirable characteristics of the radar signal and due to the fact that such techniques have not taken advantage of the positive characteristics present in the speed signals from the electromechanical sensors to complement the positive characteristics of the radar signal. Thus, such techniques have not been very conducive for providing effective tractive effort in the locomotive when the radar signal deteriorates or during periods of high acceleration or braking. For example, the foregoing variations in the radar signal could cause the calculated true ground speed of the vehicle to drop below its actual true ground speed. In this case, a controller that provides creep control in the locomotive, would erroneously reduce the tractive effort since the creep controller would estimate that the creep is higher than it actually is. The fictitious creep or wheel slip could result in unnecessary activation of the sanding valves in the locomotive with wasteful loss of the sand and the risk of equipment clogging or contamination due to the sand. Conversely, the variations in the radar signal could cause the calculated true ground speed of the vehicle to be above its actual true ground speed, and again cause the creep controller to erroneously reduce tractive effort since the controller in this case would estimate that the creep is lower than it actually is. This could result in increased locomotive vibration and reduced wheel life since the controller would allow for more creep than is permissible.
In view of the drawbacks described above, it would be desirable to provide a processor system and method for accurately determining the true ground speed of the locomotive that takes advantage of the fact that the radar signal from the available radar unit is substantially accurate when averaged over a sufficiently long period of time. It would be further desirable to provide a processor system and method that further takes advantage of the fact that the available electromechanical speed sensors, such as tachometers, provide substantially accurate instantaneous measurements of the vehicle speed. In other words, it would be desirable to provide a processor system and method configured to utilize the positive complementary characteristics of the radar signal and the respective speed signals from the electromechanical sensors to provide effective wheel slip control at all times of locomotive operation, including periods when the radar signal may be temporarily disabled, and further including periods where frequent and substantial high acceleration or braking may be expected, such as during light load operations, train yard operations, high grade climbs or descents, and the like.